Petrochemicals are recovered from boreholes drilled into the earth. When first drilled, a new borehole, commonly referred to as an open hole, is tested for the presence of petrochemicals, including tests to determine producing zones within the borehole if petrochemicals are found. Such open-hole testing is accomplished with logging tools, called open-hole tools, such as an induction tool, a sonic tool, and a neutron tool. Open-hole tools are introduced into the borehole with an open-hole truck. Assuming producing zones are identified with the open-hole tools, the borehole is prepared for production by inserting a pipe, known as a casing, into the borehole, and then setting the casing by forcing cement into the annulus between the borehole and the casing. During production, petrochemicals are pumped through the casing and recovered at the surface.
Once the well begins producing petrochemicals, it is maintained and operated with production logging tools, using a case-hole truck. Production logging tools are much less expensive than open-hole tools, and much less complex. For instance, production logging tools operate using a mono-cable, typically less than a quarter of an inch in diameter, that interfaces with a case-hole truck. In contrast, open-hole tools require a multi-conductor cable, which is a cable approximately one-half of an inch in diameter, capable of transmitting much more data from the open-hole tools to the larger open-hole truck.
During production, the casing and the cement bond holding the casing in place act as a barrier to separate oil and gas producing zones from each other, and also from water bearing strata. If the cement and casing fail to provide this separation, fluids under pressure in one zone can migrate to another zone. This can present a serious difficulty in a producing well. For instance, migration of water into a producing zone can render the well unproductive. Accordingly, once a casing and cement are inserted into a borehole, the well operator generally checks the quality of the casing and cement bond before initiating production, and periodically thereafter.
A variety of acoustic techniques have been developed to test the casing and cement bond, as are disclosed in U.S. Pat. No. 4,709,357, issued to Maki, Jr., which is incorporated herein by reference. Generally, these techniques employ a short pulse of acoustic energy against the interior of the casing, the pulse having a frequency range selected to stimulate the radial casing thickness into resonance. The returns or reflections of the pulse are received and transmitted up hole for analysis to determine the thickness of the casing and the quality of the cement bond. However, these known acoustic techniques require considerable processing to calculate casing thickness and cement bond quality, and are thus slow and expensive to employ. For instance, U.S. Pat. No. 4,709,357 by Maki, Jr. discloses a filter designed to decrease the lengthy computation time associated with the integration techniques used in analyzing reflections of short duration pulses. Further, these known acoustic techniques generally need an open-hole truck, since many measured signal values must be sent to and analyzed by a processor located up-hole outside of the borehole. The analysis can include complex mathematical equations, which can use large amounts of processing capacity and take an extended period of time.